1. Field of the Invention
The present invention relates to a device, program and method that controls the gradation of an image by controlling the transmission state of light from a light source by means of a plurality of optical transmission elements optically arranged in series, and more particularly, to a gradation control device, gradation control program and method of controlling gradation preferable for realizing expansion of luminance dynamic range and number of gradations, along with an optical display device, optical display device control program and method of controlling an optical display device.
The present application claims priority from Japanese Patent Application No. 2004-056175 filed on Mar. 1, 2004 and Japanese Patent Application No. 2004-291473 filed on Oct. 4, 2004, the content of which are incorporated herein by reference.
2. Description of the Related Art
Dramatic improvements have been made in recent years in the image quality of liquid crystal displays (LCD), EL, plasma displays, cathode ray tubes (CRT), projectors and other optical display devices, and performance with respect to resolution and color gamut are nearly comparable to human vision characteristics. However, the reproduction range of luminance dynamic range is at best about 1 to 102 nit, while the number of gradations is typically 8 bits. On the other hand, the luminance dynamic range that can be visualized all at once by human vision is about 10−2 to 10−4 nit, while luminance discrimination ability is about 0.2 nit, and when this is converted into a number of gradations, it is said to be equivalent to 12 bits. When considering the displayed images of current optical display devices in terms of these vision characteristics, the narrowness of the luminance dynamic range is conspicuous, and due to a lack of gradation of shadowed and highlighted areas, displayed images appear to lack realism and impact.
In addition, in the field of computer graphics (CG) used in movies and video games, there is a growing trend to pursue greater depiction reality by giving a luminance dynamic range and number of gradations that approach human vision to display data (referred to as high dynamic range (HDR) display data). However, due to the lack of performance of optical display devices that display that data, there is the problem in which CG images are unable to adequately demonstrate their inherent expressive capabilities.
Moreover, 16-bit color space is scheduled to be employed in next-generation operating systems (OS), resulting in a dramatic increase in the luminance dynamic range and number of gradations as compared with current 8-bit color space. Consequently, it is desirable to realize optical display devices capable of taking advantage of 16-bit color space.
Among optical display devices, liquid crystal projectors, digital light processing (DLP, trademark of the TI Corporation) and other projection-type display devices are capable of large-screen display, and are effective devices in terms of reproducing reality and impact of displayed images. In this field, the following proposals have been made to solve the aforementioned problems.
Technology for a high dynamic range projection-type display device is disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. 2001-100689. This display device is provided with a light source, a first optical modulation element that modulates the luminance of the entire wavelength region of the light, and a second optical modulation element that modulates luminance of the wavelength region for each wavelength region of three primary colors of red, green and blue (RGB) within the wavelength region of the light. In this device, light from the light source forms a desired luminance distribution by modulating with the first optical modulation element, the optical image is then formed on the pixel surface of the second optical modulation element to modulate the color, after which the secondary modulated light is projected. Each pixel of the first optical modulation element and second optical modulation element is individually controlled based on a first control value and second control value, respectively, that are determined from HDR display data. The optical modulation elements have a pixel structure or segment structure that allows independent control of the transmission factor, and a transmission factor modulation element is used that is capable of controlling the two-dimensional distribution of transmission factor. A typical example of this is a liquid crystal light valve. In addition, a reflecting modulation element may be used instead of a transmitting modulation element, and a typical example of this is a digital micromirror device (DMD).
The following considers the case of using an optical modulation element having a dark display transmission factor of 0.2% and a bright display transmission factor of 60%. In the case of the optical modulation element alone, the luminance dynamic range is 60/0.2=300. Since the aforementioned projection-type display device of the prior art is equivalent to optically arranging optical modulation elements having a luminance dynamic range of 300 in series, a luminance dynamic range of 300×300=90,000 can be realized. In addition, since the same approach is valid for the number of gradations, a number of gradations in excess of 8 bits can be obtained by optically arranging optical modulation elements having a gradation of 8 bits in series.
However, in the case of using a DMD as an optical modulation element, since the optical transmission factor, reflection factor and so forth cannot be physically changed, it is necessary to make contrivances such as changing the apparent reflection factor by controlling the reflected direction (two directions) and continuation time of the light in a the micromirror that composes the DMD with the pulse width of a control signal (PWM control). In this manner, one method that has been considered in order to realize gradation display of images by controlling the two states of transmission and non-transmission of light in a specific direction consists of generating a plurality of control signals having different pulse widths respectively corresponding to the number of gradations of each pixel of an image in the manner of a field sequential system, and using time-shared control to control the cumulative transmission time of the light to a target location according to the generated control signals.
However, in the invention described in Patent Document 1, a concrete method for realizing expansion of the luminance dynamic range and number of gradations when using DMD for the first optical modulation element and second optical modulation element is not indicated.
In addition, when gradation display of an image is realized by controlling a first optical modulation element and second optical modulation element using a field sequential system as previously described, if the first optical modulation element and second optical modulation element are synchronously controlled, since gradation display of an image is only possible at the number of gradations that can be realized with only one of the modulation elements, the number of gradations cannot be expanded using simple synchronous control.
In addition, if liquid crystal light valves are used for the first optical modulation element and second optical modulation element, since the numerical aperture ends up being about 60% as a result of building in semiconductor components and so forth, the transmission factor during the bright state as previously described is 60%, thereby resulting in a decrease in light utilization efficiency. In other words, if liquid crystal light valves of the transmitting type having a numerical aperture of 60% are used for the first optical modulation element and second optical modulation element, the numerical aperture becomes 60%×60%=36%, thereby causing the optical transmission factor to decrease to 36%.
Therefore, in focusing on the aforementioned problems of the prior art that remain unsolved, the object of the present invention is to provide a gradation control device, gradation control program and method of controlling gradation, along with an optical display device, optical display device control program and a method of controlling an optical display device, that are preferable for realizing expansion of the luminance dynamic range and number of gradations of displayed images as well as improve the light utilization efficiency by controlling the transmission state and non-transmission state of light in a specific direction in an optical transmission element.